This invention relates generally to frequency demodulators and, more particularly, to frequency demodulators for demodulating input signals that are sampled asynchronously.
Analog and digital signals frequently are transmitted over communication channels using frequency modulation (FM) encoding. Information is extracted from the modulated signals using frequency demodulators. In the case of digital FM, or frequency-shift keying (FSK), one distinct frequency represents a "0" bit and another distinct frequency represents a "1" bit. Demodulating an FSK signal does not require a precision FSK demodulator, because the demodulator merely must decide for each bit whether the signal's frequency is below or above a particular center frequency threshold. In the case of analog FM, on the other hand, a continuum of frequencies must be detected and a precision demodulator is required for the modulating signal to be accurately recovered.
In such an analog FM system, the frequency of the modulated input signal can be detected by measuring the time duration between successive zero crossings. Such time duration measurements are readily made when the FM system is a continuous time system; however, that is not the case for discrete time systems, such as systems that use digital signal processors, in which case the modulated input signal is sampled at a rate that is asynchronous with the input signal. Signal samples therefore are available only at discrete times not necessarily coincident with the zero crossings.
Discrete time FM systems in the past have used an adaptive bandpass filter that is made to search for the frequency of the input signal. The filter coefficients are repeatedly adjusted so as to maximize the amplitude of the filtered signal. Although systems of this kind have been generally effective, they are considered to require excessive processing time and complexity.
It should be appreciated from the foregoing description that there is a continuing need for a frequency demodulator for a discrete time FM system that can accurately demodulate an analog FM signal without the need for excessive data processing and its consequent delays. The present invention fulfills this need.